Creating neighbour cell lists

ABSTRACT

A neighbour cell list is formed, for use in a basestation of a cellular communications network. For each of a plurality of neighbour cells, a value of a first component is assigned, depending on whether or not the basestation can detect signals transmitted from said neighbour cell. A value of a second component is assigned, depending on a history of successful or unsuccessful handover attempts to said neighbour cell. The values of the first and second components are combined to form a weighting parameter, for use in determining a handover priority to be given to said neighbour cell in the neighbour cell list.

This invention relates to a mobile communication network, and inparticular to methods and systems whereby a cellular basestation cancreate its neighbour cell lists.

It is known to establish femtocell access points in a building, in orderto provide improved coverage for users of a cellular communicationnetwork, amongst other advantages. When a registered user device iswithin the coverage area of a femtocell access point, it can establish aconnection with that access point, with the connection from the accesspoint into the core network of the cellular network being establishedover a pre-existing broadband internet connection, for example. When theuser leaves the coverage area of the femtocell access point, theconnection can be handed over to another femtocell, or to a macrocellbase station of the cellular network.

It is also known to establish a network of such femtocell access points.

One issue that arises with all cellular communications networks is thatit is necessary for each basestation to establish a list of neighbourcells, so that each user equipment in the cell served by thatbasestation can be aware of the neighbour cells, in order to be able tochange to one of those neighbour cells when appropriate.

In the case of femtocell access points, each is responsible for creatingits own list of neighbour cells, in a way that attempts to ensure a goodpossibility of a successful handover for the user equipments, withoutforcing those user equipments to make excessive numbers of measurementson large numbers of neighbour cells.

Where there is a network of femtocell access points, for example withina single building or otherwise within a relatively small area, eachneeds to create a neighbour cell list that allows the user equipments toobtain acceptable signal quality over the whole of the intended coveragearea.

In accordance with aspects of the invention, this problem is solved byforming a neighbour cell list that includes other femtocell accesspoints within the network, As there is a high probability that some ofthe other femtocell access points will be sharing a channel and ascrambling code, a mechanism is provided for determining handoverpriorities to be given to the neighbour cells in the neighbour celllist. According to a first aspect of the present invention, there isprovided method of forming a neighbour cell list, for use in abasestation of a cellular communications network, the method comprising,for each of a plurality of neighbour cells:

assigning a value of a first component, depending on whether or not thebasestation can detect signals transmitted from said neighbour cell;

assigning a value of a second component, depending on a history ofsuccessful or unsuccessful handover attempts to said neighbour cell; and

combining the values of the first and second components to form aweighting parameter, for use in determining a handover priority to begiven to said neighbour cell in the neighbour cell list.

Thus, the handover priority to be given to a particular neighbour cellcan be determined based on whether or not the basestation can detectsignals transmitted from that neighbour cell, and on a history ofsuccessful or unsuccessful handover attempts to that neighbour cell.

According to other aspects of the invention, there are providedbasestations and networks of such basestations.

For a better understanding of the present invention, and to show how itmay be put into effect, reference will now be made, by way of example,to the accompanying drawings, in which:—

FIG. 1 shows a building in a coverage area of a cellular communicationsnetwork.

FIG. 2 shows the deployment of multiple femtocell access points in thebuilding.

FIG. 3 is a schematic illustration showing the presence of femtocellaccess points in a wider communications network.

FIG. 4 is a flow chart illustrating a first process in accordance withthe present invention.

FIG. 5 is a flow chart illustrating in more detail a part of the processshown in FIG. 4.

FIG. 6 illustrates a form of a Mobility Table constructed during theprocess shown in FIG. 5.

FIG. 1 shows a building 10, which is located within the coverage area ofa macrocell base station 12 of a cellular communications network. Thus,user devices, such as mobile phones 14, laptop computers and the like,that are in the vicinity of the building 10 can obtain a cellularservice by establishing a connection into the cellular network throughthe macrocell base station 12. The invention is described herein withreference to an embodiment in which the cellular communications networkis a UMTS network. In such a situation, it is also common that thenetwork operator will also have another cellular communications network,for example a GSM network, providing coverage to the same coverage area.In that case, there is a degree of connection between the networks suchthat a user equipment that is able to operate in both networks is ableto handover seamlessly between in them, by means of an inter-RAT (RadioAccess Technology) handover.

It is known that cellular coverage within buildings can be poor, leadingto unavailability of service, or forcing user devices to transmitsignals at high transmit powers, leading to shorter battery life.

Femtocell access points are therefore deployed within the building 10,with the intention that user devices located within the building atleast should be able to obtain a cellular service by establishing aconnection into the cellular network through one of the femtocell accesspoints.

Although the invention is described herein with reference to thedeployment of femtocell access points within a building, within whichusers are expected to circulate, such as an office building, aneducational establishment, or a shopping mall, it will be apparent thatthe invention is applicable to other situations. For example, theinvention is equally applicable to outdoor deployment of femtocellaccess points, especially but not exclusively in locations where thereis common ownership and/or management of an area in which users areexpected to circulate.

FIG. 2 is a schematic representation of one level 16 within the interiorof the building 10. In this example, the building 10 is an officebuilding, and the whole of the level 16 is occupied by a singlecorporate entity. Based on the number of expected users within the level16 at any one time, a suitable number of femtocell access points 18 aredeployed. The eight femtocell access points shown in FIG. 2 areindicated as AP1-AP8. These femtocell access points form an enterprisegroup. That is, they are managed by the single corporate entity, and areorganized such that any user equipment that is allowed to register withone of the femtocell access points is able to register with any of them,meaning that the femtocell access points within the group canself-organize, in order to improve the overall service delivery.

The femtocell access points 18 are located in suitable positions. Forexample, it may be appropriate to provide a femtocell access point closeto the or each entrance/exit point, so that users entering or leavingthe building can spend as long as possible connected to one of thefemtocell access points. In addition, the femtocell access points shouldbe distributed throughout the space, so that any user within the spacewill be able to establish a connection with one of the femtocell accesspoints.

FIG. 3 is a schematic diagram, illustrating network connections of thefemtocell access points. Specifically, the femtocell access points 18 ina group are all connected to a local area network (LAN) having a LANserver 20, which also has a connection to a wide area network 22, inparticular a public wide area network such as the internet. Thefemtocell access points 18 are able to connect over the wide areanetwork 22 to a core network 24 of the cellular communications network.The core network 24 includes a management node 26, which monitors andcontrols where necessary the operation of the femtocell access points18.

In one embodiment of the invention, the management node 26 distributesto all femtocell access points 18 in the group the relevant informationabout the group, including: the IDs of all femtocell access points inthe group; and their main RF parameters, such as the UTRA Absolute RFChannel Number (UARFCN) and scrambling code (SC), the Location Area Code(LAC) and Cell-ID, and the initial power levels.

Thus, the invention is described herein with reference to its use in anaccess point operating in accordance with existing cellular standardsset by 3GPP. However, it will be appreciated that the same techniquescan be used in networks using all existing and future networks in whichthe initial downlink power of an access point or basestation can be setbased on information available at the time.

In this embodiment, the femtocell access point can enter the downlinkmonitor mode, in which it can detect signals transmitted by otherfemtocell access points, to capture the identities of the neighbouringfemtocell access points. Thus, by matching the detected UARFCN/SC andLAC/Cell-ID transmitted by each femtocell access point with theinformation received from the management node 26, the femtocell accesspoint 18 is able to populate automatically the neighbour table. This canthen be used in the case of handovers for local mobility. Thus, mobilitywithin the group is fully supported. Cell-reselection with otherfemtocell access points is achieved by each broadcasting the relevantcarrier and scrambling code information. Handover from one femtocellaccess point to another can be achieved because each femtocell accesspoint has a full map of its neighbour femtocell access points, includingtheir IDs, and so it can send a handover command that is unequivocallypointing to a specific femtocell access point. Full support is providedfor circuit-switched (CS), packet-switched (PS) and multiple RadioAccess Bearer (Multi-RAB) call mobility, and for intra-frequency andinter-frequency handovers between femtocell access points.

In addition, each femtocell access point receives periodic measurementreports from its connected user equipments, with these reportsindicating the signal strengths of intra-frequency neighbouringfemtocell access points. Further, each femtocell access point sendsmeasurement control messages to its connected user equipments that areoperating in compressed mode, requiring them to provide periodicmeasurements of their inter-frequency neighbouring femtocell accesspoints.

Further, each femtocell access point is able to communicate with theother femtocell access points by means of the local area network towhich they are connected.

In order to be able to achieve the handovers that are necessary for eachuser equipment to obtain the intended level of service, it is necessaryfor each femtocell access point to create multiple neighbour cell lists.Specifically, it is necessary in each femtocell access point to createneighbour cell lists for user equipments that are in Idle Mode (i.e. donot have any active calls) and in Connected Mode (i.e. have at least oneactive call), and it is also necessary to create separate neighbour celllists for Intra-frequency cells (i.e. cells operating on the samefrequency as the first cell), Inter-frequency cells (i.e. cellsoperating on a different frequency from the first cell), and Inter-RATcells (i.e. cells using a different access technology from the firstcell, such as GSM cells when the first cell is a UMTS cell).

In this embodiment of the invention, the femtocell access point supportsIdle Mode mobility between different enterprise groups but supportsConnected Mode mobility only within a single enterprise group.

FIG. 4 is a flow chart illustrating in general terms the procedure thatis followed in a femtocell access point when creating its neighbour celllists. This procedure is preferably performed whenever the femtocellaccess point is powered up. The procedure can then be performed againwhenever it appears that it would produce different results, forexample, when the femtocell access point detects signals from a newnearby femtocell access point.

In FIG. 4, the process begins at step 40. As part of the startupprocedure, the femtocell access point has already selected the carrieron which it will operate, and the primary scrambling code that it willuse to identify its transmissions.

In addition, the femtocell access point receives information in the formof a Master Relationship Table (MRT). The Master Relationship Tableincludes the following information about each femtocell access point inthe group, namely: the unique Cell ID of the femtocell access point; theGroup ID of the femtocell access point; the frequency and PrimaryScrambling Code selected by the femtocell access point; the Cell ID,Primary Scrambling Code, UTRA Absolute RF Channel Number (UARFCN), CPICHTx power adjustment and CPICH Tx power of other femtocell access pointsand Macro Layer nodeBs detected by that femtocell access point; andstrongest detected cell information.

Whenever a femtocell access point powers up for the first time itbroadcasts a message to indicate that it is now part of the network. Arandom femtocell access point then sends it a copy of the MRT so that itcan start its automatic configuration.

New femtocell access points are always added into the MRT with aparticular time stamp (known as the creation time stamp). The priorityof the femtocell access point is sometimes determined by the value ofthe time stamp, as described below.

Whenever a femtocell access point changes its configuration (eitherchooses a new frequency and/or scrambling code, or updates the MobilityTable) it will rebroadcast the MRT over the local area network withthese changes. In addition, the management system may remove femtocellaccess points from the MRT if they appear to be inactive.

Further, the femtocell access point receives information obtained in itsown downlink monitor mode (DLMM). In the DLMM, the femtocell accesspoint is able to detect signals transmitted by other basestations, andis able to obtain the identity of each cell from which it is able todetect signals, and additional information such as the transmit powersused by such cells.

In step 42, the femtocell access point creates the lists of femtocellneighbours. This process is shown in more detail in FIG. 5.

Thus, in step 44 in FIG. 5, the femtocell access point creates its IdleMode femtocell neighbour cell list. To avoid coverage holes (that is, asituation where a UE can detect two femtocell access points, but the twofemtocell access points cannot detect each other), one advantageoussolution is for each femtocell access point to transmit in its neighbourcell lists all of the External and Internal scrambling codes that appearin the MRT. An external scrambling code is one which can appear in theneighbour cell lists of macro layer basestations, while an internalscrambling code is one which cannot appear in the neighbour cell list ofany macro layer basestation. The management node 26 will provide to thefemtocell access point a list of Internal and External Scrambling codesfor the enterprise, as determined by the Network Operator.

Thus, in this embodiment, the Idle Mode neighbour cell list wouldinclude all the scrambling codes (both external and internal) that areused by all enterprise groups that appear within the MRT received by thefemtocell access point. For example if two scrambling codes SC1 and SC2are used (and reported through the MRT) by femtocell access points inGroups 2 and 3 respectively, then a femtocell access point in Group 2would have both SC1 and SC2 in its Idle Mode neighbour cell list. Onlyfemtocell access points that are marked as active (i.e. not stale) inthe MRT would be incorporated in the Idle Mode neighbour cell list. Thefemtocell access point therefore scans the MRT, in order to identify anyscrambling codes that are used by any femtocell access point in anygroup mentioned in the MRT.

In step 46, these scrambling codes are added to the inter-frequency IdleMode neighbour cell list and/or the intra-frequency Idle Mode neighbourcell list, as appropriate.

In step 48, the femtocell access point creates its Internal MobilityTable, or Connected Mode neighbour cell list. More specifically, thecreation of the Connected Mode neighbour cell list involves selecting asubset of the scrambling codes that are present in the Idle Modeneighbour cell list, and then involves finding the cells (as identifiedby the combination of UARFCN, primary scrambling code and Cell ID) withthe highest priority.

In this embodiment, the Connected Mode neighbour cell list for afemtocell access point can include all the scrambling codes (bothexternal and internal) that are used by the same enterprise group thatappear within the MRT received by the femtocell access point. Forexample if two scrambling codes SC1 and SC2 are used (and reportedthrough the MRT) by femtocell access points in Groups 2 and 3respectively, then a femtocell access point in Group 2 would have SC1,but not SC2, in its Connected Mode neighbour cell list. Again, onlyfemtocell access points that are marked as active (i.e. not stale) inthe MRT would be incorporated in the Connected Mode neighbour cell list.

Thus, these scrambling codes may be a subset of the Idle Mode neighbourcell list (if the number of femtocell access points in the enterprisegroup is less than the number of available External and Internal PSCs).The femtocell access point communicates the Connected Mode neighbourcell list to its connected UEs through the measurements control message.Hence the System Information Block) SIB 12 indicator in SIB 11 indicatesthat the UE should not read SIB 12 in connected mode but rather shouldread SIB 11.

Since there could be multiple femtocell access points within anenterprise group with the same PSC, the handover destination is alsoCell ID dependant, and it is necessary to prioritize the cells so that ahandover can be triggered based on a priority order. In order to set thepriority of Cell IDs, a weighting function is calculated to determinethe most likely femtocell access point with that PSC and hence thathaving the highest priority.

FIG. 6 shows the structure of the Internal Mobility Table, containingthe Connected Mode neighbour cell list. Specifically, each entrycontains one combination of UARFCN, PSC, and Cell ID. The objective ofcreating the Internal Mobility Table List is then to rank these entriesin order of highest to lowest priority. The Internal mobility tableshould be thought of as a mapping table. It is not necessarily the casethat a primary scrambling code measured by the UE corresponds to one andonly one target cell. For example, if the UE measures PSC3 and PSC3 hasnot been identified as a neighbour before, the femtocell access point isexpected to sequentially try the femtocell access points in the sameGroup which happen to use PSC3.

The process of creating the Internal Mobility Table begins by definingthe size of the Internal Mobility Table, and setting all cells withinthe Table to zero, and then the process of populating the table canbegin. All of the cell values are set to zero at power up, in order toaccount for the fact that the femtocell access point may have beenrelocated to a different position since it was last in operation.

Firstly, the MRT is searched to find every combination of UARFCN, PSCand Cell ID that appears for the enterprise group. Other femtocellaccess points and macro layer cells are excluded from this search.However, the search does not distinguish between External and InternalPSCs. The cells that are found are then examined in more detail,depending on the neighbour relationship of the potential neighbour cellto the femtocell access point performing the process. More specifically,the cells with the more remote relationship are examined first so that,if a cell is encountered twice, the results obtained from the closerrelationship will overwrite the results obtained from the more remoterelationship.

Thus, the Relative Position weights are set for each cell found in thesearch. The relative position weight is determined by whether theneighbour is a Tier 1 Detected Neighbour (i.e. a neighbour from whichthe femtocell access point is able to detect signals when in itsDownlink Monitor Mode), a Tier 1 Reciprocated Neighbour (i.e. aneighbour that has been able to detect signals from the first femtocellaccess point, with the first femtocell access point then learning ofthis from the MRT), or a Tier 2 Neighbour (i.e. a Detected Neighbour ora Reciprocated Neighbour of a Tier 1 Neighbour). The assumption is thatthe likeliest handover candidate would be a Tier 1 Detected Neighbourand the second likeliest candidate would be a Tier 1 ReciprocatedNeighbour.

It is quite possible that the femtocell access point will be able todetect signals from two Tier 1 Detected Neighbours, i.e. two femtocellaccess points, that have the same UARFCN and PSC, but of course havedifferent Cell IDs. Since the PSC offsets of the two cells are likely tobe different, it may well be possible to differentiate between thesignals from these neighbours by applying narrow time filters around themultipath signals detected from them. Thus, the signals transmitted byone of the neighbours can be distinguished from the signals transmittedby the other, even though they are transmitted with the same UARFCN andPSC.

As part of the initialization process, relative weights k1 and k2respectively are assigned to Tier 1 Detected Neighbours and Tier 1Reciprocated Neighbours. For example k1 might take a value of 0.5, andk2 might take a value of 0.3.

First, the femtocell access point searches for Tier 2 Neighbours thatare Detected Neighbours on all UARFCNs of Tier 1 ReciprocatedNeighbours. The Relative Position Weight for all such neighbours is setto k2/4.

Second, the femtocell access point searches for Tier 1 ReciprocatedNeighbours on all UARFCNs for that enterprise group. As mentioned above,the Relative Position Weight for all such neighbours is set to k2.

Next, the femtocell access point searches for Tier 2 Neighbours that aredetected by Tier 1 Detected Neighbours on all UARFCNs of that enterprisegroup. The Relative Position Weight for all such neighbours is set tok1/4.

Finally, the femtocell access point searches for Tier 1 DetectedNeighbours on all UARFCNs for that enterprise group. As mentioned above,the Relative Position Weight for all such neighbours is set to k1.

Thus, the Internal Mobility Table is now populated with possibleneighbours, each having an assigned Relative Position Weight.

It is then also possible for the management node 26 to notify thefemtocell access point of an Operator Defined Weight for each of thepossible neighbours. The default value of the Operator Defined Weightfor every UARFCN/PSC/Cell ID combination is 0, but this parameter cantake any value in the range from 0 to 1.

The Internal Mobility Table also contains a Handover Success weight foreach UARFCN/PSC/Cell ID, and this is updated after each attemptedhandover from the femtocell access point. Thus, for each UARFCN/PSC/CellID combination, the numbers of successful and unsuccessful handoverattempts are logged.

After every handover attempt, it is determined whether the attemptsucceeded or failed. If the attempt succeeded, the current value of theHandover Success Weight parameter for that UARFCN/PSC/Cell ID isincreased. For example, the value of the Handover Success Weightparameter might be increased by 0.1, with an upper limit of 0.5.Conversely, if the attempt failed, the current value of the HandoverSuccess Weight parameter might be decreased by 0.1, with a lower limitof 0.

The Internal Mobility Table is also updated whenever a new or updatedMRT is received by the femtocell access point, and whenever anymeasurement is carried out in the Downlink Monitor Mode of the femtocellaccess point. More specifically, in these situations, the RelativePosition Weights are recalculated.

It is deduced from the MRT whether any of the entries in the existingInternal Mobility Table have become stale, in which case they areremoved, or whether there are any new UARFCN/PSC/Cell ID combinationsfor which entries need to be created. The process of recalculating theRelative Position Weights is then as described above, except that theInternal Mobility Table entries are not initially reset to zero.

The updated Internal Mobility Table is then used thereafter.

A Combined Weight is then calculated for each UARFCN/PSC/Cell IDcombination. The Network Operator can define, by means of a parameternotified to the femtocell access point, how to define the CombinedWeight.

With a first value of the parameter, the Operator Defined Weight isadded to the two Weight values calculated in the femtocell access point.Thus:

Combined Weight=Operator Defined Weight+Relative PositionWeight+Handover Success Weight.

With a second value of the parameter, the Operator Defined Weight is notused in calculating the Combined Weight. Thus:

Combined Weight=Relative Position Weight+Handover Success Weight.

With a third value of the parameter, only the Operator Defined Weight isused in calculating the Combined Weight. Thus:

Combined Weight=Operator Defined Weight.

Thus, in this embodiment, when the Relative Position Weight and theHandover Weight are used to calculate the Combined Weight, they areadded together, such that they each contribute 50% of the CombinedWeight, and this lies in the range from 0 to 1. It will however beappreciated that there are many other ways in which these variousparameters can be combined to give a weight to each UARFCN/PSC/Cell IDcombination.

Even though the Network Operator may define that the femtocell accesspoint should use only the Operator Defined Weight to calculate theCombined Weight at one particular time, the femtocell access pointshould nevertheless continue to calculate the Relative Position Weightand Handover Success Weight so that the information is current shouldthe Network Operator change the configuration.

The Combined Weight, calculated as described above, is thus used to setthe priority of each cell in the Connected Mode neighbour cell list thatis derived from the Internal Mobility Table.

Step 48 shown in FIG. 5 is thus completed, and the illustrated processreturns to step 50 in FIG. 4. The femtocell access point needs toinclude in its Idle Mode and Connected Mode neighbour cell lists cellsfrom the macro layer, and preferably the highest priority macro layercells. In this embodiment, the same macro layer cells are included inthe Idle Mode and Connected Mode neighbour cell lists, and so theprocess described below is used to find just one set of macro layerneighbours.

Thus, it has been described so far how the External and Internalscrambling codes used by the population of femtocell access pointswithin an enterprise at one particular site are extracted from the MRTto start the creation of the inter-frequency and intra-frequency IdleMode and Connected Mode neighbour cell lists.

In step 50, the process of adding macro layer neighbours to these lists,to create the complete Idle Mode and Connected Mode neighbour celllists, is started. Specifically, in step 50, the macro layer cells inthe same cellular network (i.e. in the same PLMN) that might be used asneighbours are identified.

For example, cells using scrambling codes that are part of the Internalor External Scrambling Code lists can be excluded, on the basis thatthey can be assumed to be allocated to femtocells. In this illustratedembodiment, further steps are taken to avoid identifying other nearbyfemtocells, that might be using a different range of scrambling codes,as macro layer cells. Specifically, where the femtocell access point isable to detect signals from other basestations on all available UARFCNsin its Downlink Monitor Mode, the content of their SIB 7s is alsoexamined. Since femtocell access points can use the SIB 7 to broadcastinformation about a CPICH adjustment factor that is specific tofemtocell access points, only cells whose SIB 7's do not include theCPICH adjustment factor are deemed to be macro layer cells. Thus, it isonly these filtered macro layer scrambling codes and their neighbourcell lists that are included in the macro layer neighbour rankingalgorithm.

In step 52, the macro layer neighbours that have been identified aregrouped, based on information received from the MRT and on informationobtained in the Downlink Monitor Mode, by their Radio Access Technologyand the carrier that they are using. More specifically, where thefemtocell access point is a UMTS femtocell, the macro layer cells aregrouped into UMTS macrocells using the same carrier, UMTS macrocellsusing different carriers, and GSM (or other) macrocells.

Having been grouped in this way, the neighbours are added to therelevant one of the Intra-frequency, Inter-frequency and Inter-RATneighbour cell lists, as described in more detail below. In order toidentify the highest probability neighbours, the femtocell access pointwill rely on decoded BCH information of the surrounding femtocell accesspoints in the same enterprise group, and of any macro layer cellsdetected in the Downlink Monitor Mode, and on the neighbour listinformation communicated via the MRT.

Each femtocell access point is able to obtain multiple lists ofneighbour cells, for example by decoding the lists broadcast by macrolayer and femto layer cells in their SIBs, and by examining the MRT. Thefemtocell access point then uses its measurements of the Received SignalCode Powers (RSCPs) of the detected cells, obtained in Downlink MonitorMode, in a two stage process to create its macro layer neighbour celllist through a ranking process.

In step 54, the rank for each cell is calculated.

For UMTS macro layer neighbours, the rank is calculated as follows:

Firstly, calculate a weighting constant, w_(i), for each macro layercell and each other femtocell access point detected by the femtocellaccess point in its Downlink Monitor Mode.

$w_{i} = \frac{R\; S\; C\; P_{i}}{R\; S\; C\; P_{\max}}$

where:RSCP_(i) is the RSCP value in mW of the i^(th) detected macro layer cellor femtocell access point (i=1, . . . , n), andRSCP_(max) is the largest RSCP value in mW of any detected macro layercell or femtocell access point.

Secondly, for each macro layer cell or femtocell access point detectedby the femtocell access point in its Downlink Monitor Mode, assign therelevant calculated weighting constant to each macro layer cell in theneighbour list owned by that macro layer cell or femtocell access point.

Thirdly, for each macro layer cell that appears in the MRT, assign afixed weight of 0.01. This ensures that some macro layer cells willstill appear in the final neighbour list, even in the extreme case ofall the surrounding femtocell access points and macro layer basestationsbeing blocked by the next layer of femtocell access points, but the lowweighting of 0.01 implies that these cells would in most situations betruncated from the neighbour list in favour of more likely neighbours.

Fourthly, the rank of each unique UMTS macro layer cell is calculated asthe sum of all of the weighting constants assigned to that cell in theprevious steps. That is, if a potential neighbour macro layer cellappears in the neighbour lists of more than one other macro layer cellor other femtocell access point, its rank is calculated by addingtogether the weighting constants calculated in respect of those othermacro layer cells or femtocell access points.

For 2G (e.g. GSM) macro layer neighbours, the rank is calculated in adifferent way. For each detected 2G macro layer neighbour, its rank isfirst determined by its Rx Level (Rx_Lev_(i)). Secondly for each derived2G macro layer neighbour (i.e. a neighbour that cannot be detecteddirectly by the femtocell access point performing the procedure, butthat appears in the neighbour cell list of a macro layer cell or anotherfemtocell access point detected by the femtocell access point in itsDownlink Monitor Mode), its rank is determined by the weighting constantcalculated above for the 3G macro layer cell or femtocell access pointin whose neighbour cell list it appears.

Having calculated the rank of the macro layer cells, the process nowconsiders the Intra-frequency, Inter-frequency and Inter-RAT macro layercells separately.

In step 56, considering only those potential neighbour cells on the samecarrier as the femtocell access point performing the procedure, themacro layer cells that are detected by that femtocell access point areadded to the neighbour cell list in order of their RSCP, with the cellshaving the highest RSCP being given the highest priority.

Then, in step 58, the other potential neighbour cells on the samecarrier as the femtocell access point performing the procedure are addedto the neighbour cell list in order of their rank, calculated asdescribed above.

The intra-frequency neighbour cell list has a maximum size of N_(intra),and so, in step 60, macro layer cells are selected and added to the listuntil this maximum size is reached.

In step 62, considering only those potential neighbour cells on adifferent carrier from the femtocell access point performing theprocedure, the macro layer cells that are detected by that femtocellaccess point are added to the neighbour cell list in order of theirRSCP, with the cells having the highest RSCP being given the highestpriority.

Then, in step 64, the other potential neighbour cells on a differentcarrier from the femtocell access point performing the procedure areadded to the neighbour cell list in order of their rank, calculated asdescribed above.

The inter-frequency neighbour cell list has a maximum size of N_(inter),and so, in step 66, macro layer cells are selected and added to the listuntil this maximum size is reached.

Considering the Inter-RAT, e.g. GSM, neighbours, in step 68 the macrolayer detected neighbours (that is the neighbours that can be detecteddirectly by the femtocell access point performing the procedure) areranked first. More specifically, the rank of each cell is determined byits Rx Level (Rx_Lev_(i)), with the cells having the highest receivedlevels being ranked highest.

In step 70, for each derived macro layer neighbour (i.e. a neighbourthat cannot be detected directly by the femtocell access pointperforming the procedure, but that appears in the neighbour cell list ofa macro layer cell or another femtocell access point detected by thefemtocell access point in its Downlink Monitor Mode), its rank isdetermined by the weighting constant calculated above for the 3G macrolayer cell or femtocell access point in whose neighbour cell list itappears.

In step 72, the cells are added to the lists in order of the rankdetermined above.

The inter-RAT neighbour cell list has a maximum size of N_(GSM), and so,in step 74, macro layer cells are selected and added to the relevantlist until this maximum size is reached.

There is thus disclosed a method for forming the Idle Mode and ConnectedMode neighbour cell lists for intra-frequency, inter-frequency andinter-RAT neighbours.

1. A method of forming a neighbour cell list, for use in a basestationof a cellular communications network, the method comprising, for each ofa plurality of neighbour cells: assigning a value of a first component,depending on whether or not the basestation can detect signalstransmitted from said neighbour cell; assigning a value of a secondcomponent, depending on a history of successful or unsuccessful handoverattempts to said neighbour cell; and combining the values of the firstand second components to form a weighting parameter, for use indetermining a handover priority to be given to said neighbour cell inthe neighbour cell list.
 2. A method as claimed in claim 1, furthercomprising: receiving from the network for each of the plurality ofneighbour cells a respective value of a third component.
 3. A method asclaimed in claim 2, further comprising: receiving from the network aselection signal; and determining based on said selection signal whetherthe handover priority to be given to said neighbour cell should be basedon the weighting parameter or on the value of the third component.
 4. Amethod as claimed in claim 2, further comprising: combining the valuesof the first, second and third components to form a second weightingparameter, for use in determining a handover priority to be given tosaid neighbour cell.
 5. A method as claimed in claim 4, furthercomprising: receiving from the network a selection signal; anddetermining based on said selection signal whether the handover priorityto be given to said neighbour cell should be based on the weightingparameter, or on the second weighting parameter, or on the value of thethird component.
 6. A method as claimed in claim 1, for use in afemtocell access point forming part of an enterprise group, the methodcomprising: forming said weighting parameter for each of a plurality ofneighbour cells forming part of said enterprise group; and includingsaid neighbour cells in said neighbour cell list on the basis of thehandover priority.
 7. A method as claimed in claim 6, furthercomprising: including in said neighbour cell list at least one neighbourcell not forming part of said enterprise group.
 8. A method as claimedin claim 7, wherein said at least one neighbour cell not forming part ofsaid enterprise group is a macro layer cell.
 9. A method as claimed inclaim 1, comprising: receiving a list of available scrambling codes;determining whether to calculate a weighting parameter for eachavailable scrambling code; and forming a Connected Mode neighbour celllist containing each of the scrambling codes for which a weightingparameter is calculated.
 10. A method as claimed in claim 9, furthercomprising: forming an Idle Mode neighbour cell list containing eachavailable scrambling code.
 11. A method as claimed in claim 1,comprising: attempting to detect signals from a first neighbour cell ona first channel and with a first scrambling code during a firstrecurring time period; and attempting to detect signals from a secondneighbour cell on the first channel and with the first scrambling codeduring a second recurring time period offset from the first recurringtime period.
 12. A basestation for a cellular communications network,the basestation being configured to form a neighbour cell list by, foreach of a plurality of neighbour cells: assigning a value of a firstcomponent, depending on whether or not the basestation can detectsignals transmitted from said neighbour cell; assigning a value of asecond component, depending on a history of successful or unsuccessfulhandover attempts to said neighbour cell; and combining the values ofthe first and second components to form a weighting parameter, for usein determining a handover priority to be given to said neighbour cell.13. A network of femtocell access points, connected together through alocal area network such that they can exchange information about theirrespective statuses, each being configured to create a neighbour celllist by a method as claimed in claim 1.